In the well known “lost wax” process of investment casting, a fugitive or disposable pattern, such as a wax, polystyrene or other commonly used fugitive pattern material, of the article to be cast is made by injection molding a fluid pattern material in a die corresponding to the configuration of the article to be cast. That is, the fugitive pattern is a replica of the article to be cast. In high production commercial investment casting operations, a plurality of fugitive patterns typically are attached to a central fugitive sprue and pour cup to form a gang or cluster pattern assembly. The pattern assembly then is invested in a ceramic shell mold by repeatedly dipping the pattern in a ceramic slurry having ceramic flour carried in a liquid binder, draining excess slurry, stuccoing the slurry layer while it is wet with coarser ceramic particles or stucco, and then drying in air or controlled atmosphere until a desired thickness of a ceramic shell mold is built-up on the pattern. The initial ceramic slurry and stucco layers (e.g. the initial two layers) form what is called a facecoat of the shell mold for contacting the molten metal or alloy to be cast.
Once a shell mold of desired wall thickness is built up on the pattern assembly, the pattern assembly is removed from the green shell mold typically by a thermal treatment to selectively melt out the pattern assembly, leaving a ceramic shell mold having a plurality of mold cavities with the shape of each fugitive pattern. One common pattern removal technique involves subjecting the green shell mold/pattern assembly to a flash dewaxing step where the green shell mold/pattern assembly is placed in an oven at elevated temperature to rapidly melt the pattern from the green shell mold. Another pattern removal technique involves positioning the green shell mold/pattern assembly in a steam autoclave where steam at elevated temperature and pressure is used to rapidly melt a wax pattern from the green shell mold. In the manufacture of conventional ceramic shell molds by the lost wax process using wax or polystyrene patterns can be prone to mold cracking or splitting during the pattern removal operations described above as a result of internal thermal stresses applied on the green shell mold as the fugitive pattern is rapidly heated and melted. Mold splitting or cracking must be avoided during the pattern removal process so as to produce a shell mold acceptable for receiving molten metal or alloy.
Following pattern removal, the shell mold is fired at elevated temperature to remove pattern residue and to develop appropriate mold strength for casting a molten metal or alloy. Both the investment casting process and the lost wax shell mold building process are well known, for example, as is apparent from the Operhall U.S. Pat. Nos. 3,196,506 and 2,961,751 as well as numerous other patents. The ceramic shell mold typically is cast with molten metal or alloy by pouring the molten material into a funnel-shaped pour cup of the shell mold and flowing the molten material by gravity down a sprue channel, through gates and into the mold cavities. The molten metal or alloy solidifies in the mold to form the desired cast articles in the mold cavities. That is, the cast articles assume the shape of the mold cavities, which have the shape of the initial fugitive patterns. The cast articles are connected to solidified gates, sprue and pouring cup. The ceramic shell mold then is removed, and the cast articles are cut or otherwise separated from the solidified gates and subjected to one or more finishing and inspecting operations before being shipped to a customer.
The above described lost wax investment casting process is in widespread use in casting gas turbine engine components, such as turbine blades and vanes (referred to as “airfoils”) from nickel and cobalt based superalloys. Attempts have been made in casting such components to form fugitive patterns from materials other than multi-component wax blends which are currently in widespread use. For example, attempts have been made to use molded polystyrene or polyurethane foam for practice of the lost wax process to make ceramic shell molds. However, polystyrene or polyurethane foam patterns exhibit shortcomings with respect to pattern surface quality, pattern strength, and difficulty in removal of the pattern from a shell mold invested therearound without cracking of the shell mold.